Recently, a color image has been required in the field of color copying machine and color printer. The color image forming methods having a high practical value are classified according to frequently used terms into 4 kinds, a transfer drum method, intermediate transfer method, KNC method by which plural color images are overlapped on a photoreceptor to form an color image and transferred all at once, and a tandem method.
Such methods are each named based on the different viewpoints from each other. Accordingly, a method, for example, such as an intermediate transfer-tandem method can be existed. Among these methods, a color image forming apparatus according to the intermediate transfer method has been known as one by which a high quality full color image can be obtained. In this method, color images are overlapped on an intermediate transferring member by one photoreceptor used for colors of yellow, magenta, cyan, and black or photoreceptors severally used for each color image, and transferred onto a image receiving material. Among them, a tandem color image forming apparatus in which color images are each formed by a respective color image forming unit and successively transferred, has advantage that various image receiving materials can be used, a high quality color image can be obtained and a full color image can be rapidly obtained. Particularly, the property that the full color image can be obtained at a high speed is a specific advantage.
Besides, regarding the photoreceptor, an organic photoreceptor having an organic photoconductive substance has been widely used. The organic photoreceptor has advantages such as that a material suitable to various exposing light source emitting light within the range of from visible band and infrared band can be easily developed, a material without the pollution of the environment can be selected, and the material can be produced with a low cost. The only one drawback of the organic photoreceptor is that it is weak in the mechanical strength and the frictional abrading and scratches tends to be occurred on the surface thereof when a lot of copies or prints are made.
It is proved that an image defect such as divergence of the registration is occurred in the color images overlapped on the intermediate transfer member since the frictional coefficient between the surface of the intermediate transfer member that of the photoreceptor is varied under a condition with a high temperature and a high humidity, when such the photoreceptor is applied for an image forming apparatus using the intermediate method. It is studied that the use of a photoreceptor or a protective layer each containing a durable organic silicone resin binder as a countermeasure against such the problem. The advantage of such the binder is that the initial layer thickness is kept after the use of a long period since the frictional abrading of the photoreceptor is very low. However, problems of blur and flowing of image caused by filming of toner or paper powder tends to be occurred. Such problems tend to be amplified in the image forming apparatus using the intermediate transfer member.
In the tandem system, four image forming units of yellow, magenta, cyan, and black are usually necessary. In such the system, the photoreceptor used in the image forming unit is abraded by a member contacted or almost contacted with the photoreceptor such as a cleaning blade, a transfer belt, a developer, and a releasing means, and the degree of the abrasion of each of the photoreceptors is not uniform. One reason of such the phenomenon is that the influence of the toners to the photoreceptor are different form each other. When the photoreceptor is repeatedly used, the charging properties of each of the toners are considerably different depending on the difference of the abrasion accompanied with the increasing the repeated times. Such the fact undesirably influences to the color balance of the finished color image. Besides, when a toner having a high moisture content is used, the toner tends to be remained on the abraded surface of the photoreceptor. Accordingly, a part of the toner is repeatedly passed the cleaning process and a film of the toner is formed on the all surface of the photoreceptor by the effect of the pressure of the cleaning means such as a cleaning blade. The formation of the toner filming causes the blur or flow of image. When the degree of such the image blur is different from each other drums, the color balance of the image is broken and the image resolution ability is lowered. As a results of that, such the apparatus cannot be accepted as the commercial product. The toner filming can be prevented by the use of a photoreceptor having an easily abrasive surface so that the surface of the photoreceptor is abraded in some degree by the cleaning means such as a cleaning blade. However, such the countermeasure increases the difference of the abrasion levels of each of the photoreceptors used in each of the image forming unit and results degradation of the color balance.